Such a multicast packet generation device of a packet switching telecommunication system is already known in the art, e.g. from the article "Design of a Broadcast Packet network"of J. S. Turner published in the proceedings of IEEE INFOCOM'86--Fifth annual conference, Apr. 8-10, 1986 on "Computers and Communications Integrations, Designs, Analysis, Management", pages 667 to 675. Therein, the telecommunication system includes a multicast packet generation device which is constituted by a packet copy network and a plurality of broadcast and group translators.
The packet copy network has an architecture which is similar to that of a classical telecommunication packet switching network in that it has a plurality of inlets, a plurality of outlets and includes a plurality of interconnected switching elements. The inputs of the telecommunication system are coupled to respective inlets of the copy network and the outlets of the latter are each connected to a distinct broadcast and group translator. The output of each translator is connected to a distinct inlet of the switching network which is, in this case, constituted by the series connection of a packet distribution network and a packet routing network.
The difference between a classical switching network and the copy network is that the latter replicates any single original packet received at one of its inlets, i.e. at one input of the telecommunication system, and transfers each of these replica packets to a distinct one of its outlets, i.e. to a distinct broadcast and group translator. Moreover, the replica packets are merely distributed to all the outlets of the copy network. Since the replica packets received in the different translators are then all identical, each of these translators must either merely discard the replica packet if the latter is not needed or translating packet control data contained in the header of the replica packet into a packet identification and a routing information. The replica packets discarded by the translator are replica packets generated in excess by the copy network which, as already mentioned, systematically generates a maximum number of replica packets, this number corresponding to the number of outlets of the copy network and thus also to the number of translators. The packet identification is indicative of the communication, i.e. the input-output connection, to which the packet belong, and the routing information is indicative of the route to be followed by the packet through at least the switching network in order to exit at a single output of the telecommunication system.
The packet identification and the routing information form part of new packet control data which is associated by the translator to the replica packet. More particularly, this new packet control data is loaded in the header of the replica packet. The so-modified replica packet is then called multicast packet.
It is to be noted that each packet also contains user data and that this user data is the same for an original packet and all the multicast packets derived therefrom.
It is also to be noted that beside the routing information, the packet identification is necessary for distinguishing multicast packets which have to be transferred to a same output of the telecommunication system, i.e. which have the same routing information. This is for instance the case when a telecommunication concentrator is connected to this output and performs distribution of the packets, e.g. TV distribution, to different users connected to this concentrator.
A first drawback of this known multicast packet generation device is that the generation by the copy network of unneeded replica packets leads to unnecesary high traffic load.
Additionally, the control means of this known multicast packet generation device and more particularly of the broadcast and group translators thereof is relatively complex since it has to coordinate the operation of all these translators. Indeed, as mentioned above, each translator has either to discard the replica packet received therein or to translate the packet control data into particular routing information and packet identification.
Furthermore, the maximum number of multicast packets which will ever be requested has to be estimated for determining the number of outlets of the copy network. Because the copy network of the multicast packet generation device always generates this maximum number of replica packets, the same maximum number of translators are required as well as the same maximum number of inlets of the switching network needs to be reserved for performing multicast connections even when this maximum amount of multicast packets is not requested.